NEXUS-4997: SMTP SSL settings used incorrectly

Plain/SSL/TLS are actually exclusive settings
(only one can be used at a time), but the UI
used checkboxes for SSL and TLS making one think
they might be combined.

Also, the labels were misleading.
NEXUS-4997: Connection combo for SSL/TLS
Merge pull request #126 from sonatype/nexus-4997-smtp-ssl

NEXUS-4997: SMTP SSL settings used incorrectly